The 5C8 antibody reacts with human CD154 which is also known as CD40L (CD40 ligand). CD154 is a 39kDa member of the TNF-α superfamily and is predominantly expressed on activated T cells but is also expressed on NK cells, mast cells, basophils, and eosinophils. CD154 binds to CD40, a molecule that is expressed on antigen presenting cells including B cells, monocytes/macrophages, and dendritic cells. The interaction of CD154 and CD40 regulates the activation of antigen presenting cells and is critical for B cell costimulation. The 5C8 antibody has been reported to block CD40L-CD40 interactions.